Is it safe to switch impedence selector while amp is on....

Yesterday I hooked up two cabs (16 ohms each) to my amp, set at 8 ohms. After a while I felt like only using 1 cab, so I put the amp on standy, power still on, unhooked one cab and switched the impedence knob to 16. Took it off standy, sounded fine.

Nothing happened, I'm just wondering if this practice will eventually wear something out like the tubes or whatever.

It is safe to switch an impedence selector with the amp "off", or in "stand by". The only risk is when the amp is "on" AND you are playing through it at the same time. Some switches will momentarily open the circuit when switching and, if you are playing through the amp with no load on it for that short instant, the stored energy in the output transformer could reflect back into the amp and cause damage. However, if you're not playing through the amp (no sound coming from the amp), switching the impedence selector with the amp "on" won't hurt anything because there's no signal present to be reflected back into the amp.

Hey Vaughn, how's it hangin! Thanks for the info. I just bought my first Marshall and that was something I'd wondered about.

Just found this place and realized who you are. Thanks again for the repairs on my TS9. It's my favorite pedal of all time....and the only one that's safe from the auction block! Hey let me know when you start marketing your new pedal ok? You can PM me here.

HeeHaw, you must have some brave friends. Pulling the speaker cable out of running amp is crazy. The whole time that output transformer isn't seeing a load, it is cooking!!! Only change impedance or speakers while a amp is off. I would recommend you check your amp's circuitry/schematic before blanketly saying that it is safe to do this while the amp is on standby.

Yesterday I hooked up two cabs (16 ohms each) to my amp, set at 8 ohms. After a while I felt like only using 1 cab, so I put the amp on standy, power still on, unhooked one cab and switched the impedence knob to 16. Took it off standy, sounded fine.

Nothing happened, I'm just wondering if this practice will eventually wear something out like the tubes or whatever.

thanks

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On stby WITHOUT a signal (your guitar playing) thru the amp, No problem at all!!

Originally posted by RGB Hey Vaughn, how's it hangin! Thanks for the info. I just bought my first Marshall and that was something I'd wondered about.

Just found this place and realized who you are. Thanks again for the repairs on my TS9. It's my favorite pedal of all time....and the only one that's safe from the auction block! Hey let me know when you start marketing your new pedal ok? You can PM me here.

....we now return you to the previous thread.

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Hi Rick, nice to see you around these parts. Glad to hear the TS9 is working out for you!! Just trying to clear up a few amp myths!

Originally posted by TheGrooveking HeeHaw, you must have some brave friends. Pulling the speaker cable out of running amp is crazy. The whole time that output transformer isn't seeing a load, it is cooking!!! Only change impedance or speakers while a amp is off. I would recommend you check your amp's circuitry/schematic before blanketly saying that it is safe to do this while the amp is on standby.

TheGrooveking

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Output transformers don't "cook" with no load on them....they just sit there waiting to pass a signal. And, with no signal present, they just sit idle.

Pulling the speaker cable out of a running amp is safe as long as there's no signal passing through it. There has to be a signal present for voltage to be reflected back into the amp to do any (flyback) damage. Besides, practically every tube amp out there has shorting type speaker jacks that will drain any energy to ground when a speaker cable is unplugged. However, if you have a head & cab, unplugging the speaker end of the cable can be risky because that creates an open circuit condition....but, again, there has to be as signal going through the amp at the time for any damage to occur.

Originally posted by TheGrooveking HeeHaw, you must have some brave friends. Pulling the speaker cable out of running amp is crazy. The whole time that output transformer isn't seeing a load, it is cooking!!! Only change impedance or speakers while a amp is off. I would recommend you check your amp's circuitry/schematic before blanketly saying that it is safe to do this while the amp is on standby.

TheGrooveking

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I've always been so paranoid about turning on a tube amp w/out a load. I ALWAYS check the speaker cable, you never know if someone or somehow it got pulled out.

Does anyone know about how much time you have from when you turn on a tube amp until damage is done to your OT? If I ever had a bad cable, blown speaker or bad connection and I fired up the amp and started playing and no sound came out would it be too late?

Originally posted by rastaman I've always been so paranoid about turning on a tube amp w/out a load. I ALWAYS check the speaker cable, you never know if someone or somehow it got pulled out.

Does anyone know about how much time you have from when you turn on a tube amp until damage is done to your OT? If I ever had a bad cable, blown speaker or bad connection and I fired up the amp and started playing and no sound came out would it be too late?

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Time doesn't matter. A normally functioning tube amp with no load (or even a short), with no input signal applied, will essentially sit there forever with no damage. However, if you apply a signal to (play through) the amp under no load conditions, and the volume control is up high enough to create a flyback voltage in the output transformer, damage could be instantaneous or none at all. Some transformers just have better insulation than others.

A blown speaker that has an open coil can create an open circuit condition if it's the only speaker connected to an amp or if it's in series with another speaker. However, with speakers in parallel, there will still be a load on the amp. So, series connected speakers are riskier with a tube amp than parallel connected speakers.

A shorted speaker cable will stress the output tubes but will unlikely hurt an output transformer because the tubes are self current limiting. An open speaker cable or a bad (open) connection may or may not damage a tube amp, depending on the conditions I mentioned above.

Thanks VaughnC! If it happened and you turned the amp off immediatly, then you connected a load and turned it back on could you tell if there was any damage? Even if everything sounded normal? It sounds like the OT could start to heat up considerably and I was wondering if you would ever know if you damaged it.

Originally posted by rastaman Thanks VaughnC! If it happened and you turned the amp off immediatly, then you connected a load and turned it back on could you tell if there was any damage? Even if everything sounded normal? It sounds like the OT could start to heat up considerably and I was wondering if you would ever know if you damaged it.

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Unless the amp is doing something strange, blows a fuse, or sounds different, chances are you didn't do any damage. However, if the transformers insulation has been slightly damaged by flyback voltage, I suppose it's possible that a problem may not be immediately apparent.